Conventional electro-optical switches can be realized using a number of different waveguide, electrode and substrate orientations. Two different designs are used in commercially available electro-optical switches; the Mach-Zehnder and the .DELTA..beta. directional coupler. The Mach-Zehnder interferometer utilizes two 3-dB directional couplers, where the first 3-dB coupler splits the incident signal into two signals, ideally of equal intensity. If a differential phase shift is introduced between these signals, then when they re-combine in the second 3-dB coupler, the ratio of power in the two outputs will be altered. Contrast ratios greater than 20dB (e.g., 100:1) are routinely achieved in commercial devices. In the .DELTA..beta. directional coupler switch, electrodes are placed directly over the coupler and an applied electric field functions to alter the power transfer between the two adjacent waveguides. The contrast ratios achieved with the .DELTA..beta. directional coupler switch are comparable to those of the 3-dB coupler arrangement.
A "mirror" switch can be defined as an arrangement including a pair of bidirectional ports. In a first state of the mirror switch, the ports are directly coupled together (a "pass through" state). In a second state (hereinafter referred to as the "reflective" state), the ports are de-coupled so that an input signal is directly reflected and then returned back through the same port, that is, an optical signal input into a first port would be reflected back into that first port and, possibly, an optical signal input into the second port would be reflected back into the second port. If the second port is reflected back to itself also, the mirror switch is defined as "complete"; if the second port is not reflected back to itself, the mirror switch is "incomplete".
The successful design of optical apparatus frequently depends upon selection of materials having appropriate physical characteristics such as refractive index. Birefringent materials; that is, materials in which different polarizations have different refractive indices, have been known for a long time and have been used in optical apparatus. An important class of birefringent materials is formed by uniaxial materials; that is, materials with a single optic axis. As is well know, there is no birefringence for light rays parallel to the optic axis. In fact, such birefringent materials exist in nature with calcite probably being the best known. Many people have observed the double image that results when a piece of calcite is placed over an object. This is understood by considering a parallelepiped solid formed of a uniaxial birefringent material and a light beam perpendicularly incident on one surface of the material. One polarization forms the ordinary ray (the "O" ray) which is refracted in a manner that is independent of the orientation of the optic axis at the surfaces; this ray passes directly through the solid as if the solid were isotropic. The other polarization forms the extraordinary ray (the "E" ray) which is refracted in a manner dependent upon the relative orientation of the optic axis at the surfaces and the incident ray direction; the extraordinary ray emerges from the solid parallel to the ordinary ray but spatially displaced from it. Thus, there are two separate images at the output.
Although the double image observed when calcite is placed over an object is probably most used as a laboratory demonstration in elementary science courses, birefringence is now widely used in certain types of optical devices. Because one beam moves away spatially from the other beam, the devices are commonly referred to as "walk-off" devices. The spatial separation of the beams increases linearly as the beams pass through the birefringent material. The subsequent, independent separate processing of the beams is most easily performed if the spatial separation of the beams is large. However, large separation of the beams requires a long piece of birefringent material.
It is noted that the term "walk-off" is not used to describe beam splitters; these devices only use reflection to obtain spatial separation and spatial redirection of beams. Birefringent based devices are typically effective over a broader band of wavelengths than are beam splitters and produce better polarization discrimination than do beam splitters. Conversely, walk-off devices can be used to overlay parallel beams of orthogonal polarizations into a single beam of both polarizations.